Concentrated hydrochloric acid with a concentration greater than 10 wt. % is often contaminated by heavy metals. Such heavy metal contaminants normally have to be removed before the acid is used further.
Such heavy metal contaminants are known to be removed by contact with anion exchangers, since the heavy metals are, for the most part, present as chloro-complexes. The loaded ion exchangers are then regenerated by treatment with water or bases. During such a regeneration treatment operation, the heavy metal anions are replaced by hydroxide ions and removed from the ion exchanger.
Generally, in cases where the heavy metal contaminants include Sn(IV), the tin is present as a hexachlorostannate complex, which is only stable at high hydrochloric acid concentrations. It is not possible to regenerate the loaded ion exchangers using water, since the tin forms sparingly soluble tin hydroxide, which is precipitated in the ion exchanger.
An Sn(IV)-contaminated ion exchanger can be regenerated with dilute sodium hydroxide solution (concentration approximately 1 to 15 wt. %). The Sn(IV) is converted to a soluble hexahydroxystannate complex. During regeneration with sodium hydroxide solution, however, a large amount of heat of neutralization is released. In addition, a large amount of fresh water has to be used to rinse the ion exchanger.